This work consists of two projects which explore the relative contributions of axial streaming and laminar Taylor dispersion to the longitudinal mixing of respired gases during inspiration and during expiration. Moreover, we will study the effect of turbulence, that is generated in the upper airways, upon these processes. Mixing will be measured by monitoring both the inspired and expired concentration-volume curves generated by a 20 ml bolus of helium or sulfur hexafluoride which is automatically introduced into inspired air; the extent of mixing is expressed as the difference between the variances of these two curves. In Project I, a 5-generation symmetric glass model and a positive cast of the lung will be prepared; both will be designed such that a tube constriction can be placed in the tracheal branch. The extent of mixing of oxygen and of sulfur hexafluoride, the mean gas velocity, and the turbulence intensity will be measured in progressive branches (in both tube networks) as a function of inspiratory-like flows in the range 0.2 to 4 lambda/s; these measurements will be repeated for two tube constrictions of varying sizes placed in the tracheal branch. The difference in the extent of mixing caused by insertion of the constriction will be correlated with the turbulence intensity. In Project II, the extent of mixing and the breath sound intensity will be measured in five normal subjects as a function of inspiratory flow and of expiratory flows in the range 0.04 to 4 lambda/s.